Decorative finish for copper



United States Patent 3,284,249 DECQRATIVE FINISH FOR COPPER Donald H. Osborn, Naugatuck, Conn, asslgnor to Anaconda American Brass Company No Drawing. Filed June 28, 1963, Ser. No. 291,237 8 Claims. (Cl. 148--6.24)

This invention relates to a method of imparting a decorative finish on copper and its alloys More particularly it relates to the method of imparting a decoratlve statuary bronze finish on a cupreous surface, and to products on which the decorative finish is formed.

Normally when copper and its alloys are exposed to the natural atmosphere, their surface colors w ll gradually change according to the copper composltion and the atmospheric conditions. In this natural weathering of copper, the color changes gradually from the bright metallic pink of the new metal through red brown, dark brown, ebony, grays, and gray greens, to the ultimate blue green patina. The mellowing of the copper base alloys follows an analogous pattern, but the time rates of the change are different and the colors are not quite the same as those from an unalloyed copper.

The gradual change of colors :on the cupreous surface is due to the slow formation of a relatively thin surface film of copper compounds produced by the chemical reaction of the cupreous surface with atmospheric constituents such as oxygen, carbon dioxide, hydrogen sulfide, sulfur dioxide, and sodium chloride in the presence of atmospheric humidity, dew and rain. Depending on the composition of the cupreous surface, a range of pleasing brown colors, typically that of a well-kept bronze statue, will occasionally appear on the surface after a relatively long period of exposure in the natural atmosphere. The metals having these colors are very pleasing in appearance and are desirable in many architectural and decorative applications. The length of time required, and the difficulties encountered in obtaining this pleasing brown finish on the cupreous surface, however, renders the natural process highly impractical. Thus, many artificial methods using chemicals or mixtures of chemicals were pro-posed to provide the cupreous surface with the pleasing and stable statuary bronze finish in a short length of time.

The chemicals or mixtures of chemicals used in the prior methods are intended to react with the cupreous surface to provide a surface film of copper compounds. The color of this surface film varies considerably depending on the chemicals used and the copper composition of the cupreous surface. However, precise control in obtaining or duplicating tone and depth of color is not attainable in the prior art. Invariably all the films imparted on the cupreous surfaces require additional work such as hand toning or repeated treatment with chemicals to obtain the aesthetically effective shade.

The chemical compositions of these surface films generally can be classified into three categories: (1) films consisting of predominantly copper oxides, (2) films consisting of predominantly copper sulfides, and (3) films consisting of predominantly selenium compounds.

The oxidic films, which can be imparted on a cupreous surface readily, lack color uniformity and aesthetical ap pearance, and in addition, the colors of these films are diflicult to reproduce and tend to darken rapidly when exposed to natural atmosphere. The physical softness of these films further increases the problems of maintaining the colors as produced, thus rendering them even less desirable. Thin sulfidic films that have the required aesthetically effective shade, on the other hand, are difficult to produce on a cupreous surface. Strong acid is normally required to etch the cupreous surface prior to sulfidizing, and the heavy black sulfidic films produced on the cu reous surface require considerable subsequent hand toning in order to produce the needed aesthetical shade. The films consisting of selenium compounds are characterized by similar disadvantages as the oxidic and. sulfidic films, and the inherent high cost of selenium compounds further limits their commercial value.

The present invention provides a process by which the disadvantages of the prior processes will be overcome, and the finish that is provided on the cupreous surface will be uniform and pleasing in appearance. The process of this invention can be controlled precisely to impart a decorative finish that has the predetermined tone and depth of color. The finish is tightly-adherent and has the desired statuary bronze finish, i.e. a brown bronze-like color, characteristics similar to coatings formed naturally on the cupreous surface after a long period of exposure to the atmosphere.

Broadly stated, the method of the invention consists of treating a cupreous surface with potassium permanganate to form discrete adherent microscopic particles of predominately cupreous oxide uniformly distributed on said surface and spaced apart at a distance greater than the dimension of the particles. The treatment is continued for a controlled period of time allowing the oxide particles to grow to cover a predetermined fraction of the total area of the cupeous surface. This oxide is then reacted with an aqueous ammoniacal sulfide solution, whereby a decorative statuary bronze finish is provided on the cupreous surface with a predetermined depth of color.

The finish of the invention can be imparted on the cupreous surface of an article to provide a decorative statuary bronze finish on at least a portion of its surface, which comprises discrete adherent particles covering a major fraction of the total area of the cupreous surface. Each of these particles substantially is intermixed cuprous oxide and copper sulfide molecules. These adherent particles are essentially oxidic at the interface between the particles and the cupreous surface, and essentially sulfidic at the surface of the particles. These particles of intermixed oxidic and sulfidic molecules provide aesthetically statuary bronze colors on the cupreous surface.

In one example, a cupreous surface to be finished was first degreased and thoroughly cleaned to remove all the undesirable surface dirts to insure uniform coloring. The cleaned surface was then treated with an aqueous oxidizing solution comprising a strong oxidizing agent, namely potassium permanganate and an electrolyte, preferably copper sulfate. The preferred range of composition of this solution is:

Water to make one liter.

It was found this solution reacts with cupreous surface to form discrete and adherent microscopic oxide particles uniformly distributed on the cupreous surface and spaced about 0.001 to 0.01 inch apart. By allowing the reaction to continue, the oxide particles will grow to cover substantially the entire area of the cupreous surface. This reaction is termed here as nucleate oxidation for the purpose of describing this invention. Electron diffraction analyses of these oxide particles showed cupreous oxide (Cu O) to be the predominant constituent. This solution can be used and re-used as long as the only consumable constituent of the solution, potassium permanganate, is maintained at its specified concentration range. The depletion of the permanganate tends to lower the pH of the solution, and when the pH drops below 2.8, the solution will have an undesirable etching effect and the oxidic particles forming property of the solution will be destroyed. Thus, it is important to replenish the permanganate periodically to maintain the pH level of the solution between 3 3.1 to 4.1, preferably in the range of 3.5 to 3.8. If addition of the permanganate fails to rest-ore the pH to a proper level, the old solution should be discarded. The upper limit of the solution should also be observed to prevent the precipitation of copper sulfate as a basic salt.

Subsequent to the formation of oxide particles by nucleate oxidation, the treated cupreous surface is rinsed to remove loose deposits. It is desirable to dry the surface rapidly after rinse to avoid staining or spotting. Slight brushing of the treated surface sometimes is advantageous to tone up the color and at the same time to eliminate loose scum or sludge of manganese dioxide. After removal of loose deposits, the oxide is now in condition for sulfidizing.

An aqueous ammoniacal sulfide solution comprising mixtures 'of ammonium sulfides and ammonium polysulfides has been found to be most effective to sulfidize the film of oxide particles.

The aqueous ammoniacal sulfide solution, which does not attack the cupreous surface readily, will react with the oxide particles rapidly to form a complex mixture of oxide and sulfides. Sodium hydroxide or other chemicals can be added to maintain the basic pH in the solution and to prevent the precipitation of sulfur. Any amount of sodium hydroxide or other chemicals effective for this purpose can be used. A surface active agent such as glycerine may also be added into the sulfidizing solution to adjust the density, viscosity and surface tensions of the liquid to permit the solution to flow more evenly over the surface and thereby improve the uniformity of the color. Care must be taken in incorporating acidic surface agents such as glycerine into the solutions, so that they will not precipitate the sulfur present in the solutions. It is usually desirable to incorporate this acidic surface active agent in the solutions after the addition of alkaline hydroxide in order to prevent the lowering of the pH. The preferred composition of the solutions that contains surface active agents glycerine and sodium hydroxide is:

Ammonium sulfide (NHQ S (light) ml 4-6 Ammonium polysulfides (NI-1.0 5,; (dark) ml 4-6 Glycerine ml -10 1 M NaOH ml -35 Water liters l Triethanolamine may be substituted for the glycerine in the above formula, in which case the amount of sodium hydroxide may be considerably reduced or dispensed with altogether. The polysulfidic solution is a standard reagent solution which assays 12% sulfur. The range of sulfide concentration is selected to give the best statuary bronze finish on the cuprous surface. S'ulfidic concentrations much above the upper limits will have a leaching effect which diminishes the color forming property of the solution. Sulfidic concentration can be as low as a half milliliter and still impart a statuary finish on the surface. However, a concentration, much lower than the preferred range, is not desirable because the time required to form the finish increases to an impractical length, and the solution may not be strong enough to attack the oxide to form a good statuary bronze finish.

The exact mechanism of reaction between the sulfidizin-g solution and the oxide is not known. Electron diffraction analyses of the coloring film showed a large fraction of cup-rous oxide converted to give a mixed composition of cuprous and cupric sulfides. The formation of the complex finish on the cupreous surface may be visualized as occurring in the following steps:

As shown by the standard-free energy change (AF.) for reaction (1), than is a fairly strong tendency for cuprous oxide and the ammonium radical to form the cuprammonium complex. However, the relatively large equilibrium constant for reaction (2) indicates that this complex breaks up quite readily, and leaves copper ion free to combine with sulfide ion. The oxidation of cuprous to cupric ion is due to the strongly oxidizing nature of the polysulfide compounds. The small equilibrium constants of reactions (3) and (4) show that the greater tendency is for both of these reactions to proceed in the reverse direction to form the sulfides.

The unconverted oxide remains mostly at the diffused interface between the cupreous surface and the particles to form a strong bond for the coloring finish. The concentration of this oxide diminishes gradually as the depth of the surface decreases, with only the abrasive-resistant, essentially mixed sulfide compounds remaining at the surface of the particles. Thus, these complex particles give an excellent statuary bronze finish from the standpoints of adherence, abrasive resistance, stability and uniformity of colors, and in its generally pleasing appearance.

The color and the shade of this finish is determined by the composition and the fraction of the total surface covered by the coloring particles, which are in turn dependent on the original composition of the cupreous surface, the temperatures, the concentrations of the solutions, and the contact times of the solutions with the cupreous surface. These factors can be effectively altered to give the most desirable color and shade of the final finish. In order effectively to control the variables to insure reproducibility, the temperature of the solutions and the contact times of the two separate treatments are preferably within limited ranges. The preferred range of temperature for the oxidizing solution is 50 C. to C. with contact time of one to five minutes depending upon the alloy and the color desired. The temperature of the solution and the contact time for the sulfidizing treatment is not too critical; a temperature range of 20 C. to 50 C. and with a contact time range of 15 seconds to one minute are quite satisfactory. The contact times of the sulfidizing treatment can be extended to above two minutes or shortened to below 15 seconds without causing noticeable change in the final color. In order to prevent vaporization of the ammonia in the solution, the temperature should not be much above 50 C., and to insure that the reaction is not reduced to an undesirably slow rate, the temperature should not be much below 20 C.

It has heretofore been thought that a longer contact time and a higher temperature in the sulfidizing treatment will give a darker color at the final finish. On the contrary, it has been found that varying the contact time and the temperature of the sulfidizing treatment will not appreciably affect the final color, so long as the contact time and the temperature are within the stated range. Thus, using the preferred compositions of the two solutions, the color of the final finish can be best controlled by only adjusting the contact time or the temperature of the oxidizing treatment, to obtain a proper oxidic particles density and the desired statuary bronze finish. The simplicity in controlling the finish to insure easy reproducibility of a uniformly colored finish is one of the important features of this process.

The statuary bronze coloration imparted on the eupreous surface in accordance with this invention is sufficiently stable under normal atmospheric conditions. However, if the exposure condition is such that it causes the finish to continue to turn green, a transparent coating on the finish provided by oiling, lacquering or other forms of surface protective treatment is desirable. When the statuary bronze finish on the cupreous surface requires restoration, pickling or other types of chemical and mechanical treatments can be used to restore the copper base, on which a new statuary bronze finish can be imparted by the method of this'invention.

I claim:

1. A method of imparting a decorative statuary bronze finish to a cupreous surface which comprises treating said surface with potassium permanganate, thereby forming discrete adherent microscopic particles of predominately cuprous oxide uniformly distributed on said surface and spaced apart at a distance greater than the dimension of said particles, continuing said treatment for a controlled period of time, whereby said oxide particles grow to cover a predetermined fraction of the total area of said surface, and thereafter reacting said oxide with an aqueous ammoniacal sulfide solution whereby a decorative statuary bronze finish is provided on said surface with a predetermined depth of color.

2. A method of imparting a decorative statuary bronze finish to a cupreous surface which comprises treating said surface with an aqueous potassium permanganate oxidizing solution thereby forming discrete adherent microscopic particles of predominately cuprous oxide uniformly distributed on said surface and spaced apart at a distance greater than the dimension of said particles, continuing said treatment for a controlled period of time, whereby said oxide particles grow to cover a predetermined fraction of total area of said surface, and thereafter reacting said oxide with an aqueous ammoniacal sulfide solution, said sulfide containing at least one sulfur atom in each molecule, whereby a decorative statuary bronze finish is provided on said surface with a predetermined depth of color.

3. A method of imparting a decorative statuary bronze finish to a cupreous surface which comprises treating said surface with an aqueous oxidizing solution comprising potassium permanganate and copper sulfate thereby forming discrete adherent microscopic particles of predominately cuprous oxide uniformly distributed on said surface and spaced apart at a distance greater than the dimension of said particles, continuing said treatment for a controlled period of time, whereby said oxide particles grow to cover a predetermined fraction of total area of said surface, and thereafter reacting said oxide with an aqueous ammoniacal polysulfide solution, whereby a decorative statuary bronze finish is provided on said surface with a predetermined depth of color.

4. A process of imparting a decorative statuary bronze finish on cupreous surface, the combination of steps which comprises (a) cleaning said surface,

(b) treating said surface with a strong oxidizing solution of potassium permanganate and copper sulfate having a pH range of 2L8 to 4.1,

(c) removing the loose residual deposits of said oxidizing solution from said surface, and

(d) subjecting said surfaces to the reaction of an aqueous ammonium polysulfide solution having a pH range of 8.0 to 10.0,

whereby a decorative statuary bronze coloration is provided on said surface.

5. A process of imparting a decorative statuary bronzelike finish on a cupreous surface, the combination of steps which comprises (a) cleaning said surfaces,

(b) treating said surfaces with an oxidizing solution consisting essentially of:

Gms. Potassium permanganate (KMnO 4 to 6 Copper sulfate 40 to 60 Water to make one liter.

(c) removing loose deposits from said surfaces, and (d) subjecting said surfaces with a polysulfide solution consisting essentially of:

M1. Ammonium sulfide 4-6 Ammonium polysulfide 4-6 Glycerine O-10 Sodium hydroxide 10-35 Water to make one liter.

whereby a continuous adherent decorative film is provided on said surface.

6. A process of imparting a decorative statuary bronzelike finish on a cupreous surface, the combination of steps which comprises (a) cleaning said surfaces,

(b) treating said surfaces with an oxidizing solution consisting essentially of:

Gms. Potassium permanganate (KMnO 4 to 6 Copper sulfate 40 to 60 Water to make one liter.

(c) removing loose deposits from said surfaces, and (d) subjecting said surfaces with a polysulfide solution consisting essentially of:

M1. Ammonium sulfide 4-6 Ammonium polysulfide 4-6 Triethanolamine 0-10 Water to make 1 liter.

whereby a continuous adherent decorative film is provided on said surface.

7. A cupreous article having a decorative statuary bronze finish on at least a portion of its surface, said finish comprising discrete adherent particles covering a major fraction of the total area of said surface, each of said particles substantially being intermixed cuprous oxide and copper sulfide molecules, said adherent particles being essentially oxidic at the interface between said particle and said surface and essentially sulfidic at the surfaces of said particles.

8. A cupreous article having a decorative statuary bronze finish on at least a portion of its surface, said finish comprising discrete adherent particles covering a major fraction of the total area of said surface, each of said particles substantially being intermixed cuprous oxide and copper sulfide molecules, said adherent particles being essentially oxidic at the interface between said particles and said surface and essentially sulfidic at the surface of said particles, and a transparent protective coating on said finish whereby a permanent decorative statuary bronze-like finish is provided thereon.

References Cited by the Examiner UNITED STATES PATENTS 1,951,304 3/ 1934 Freeman 148-6.24 2,054,737 9/ 1936 Brunner 148-6.24 2,288,788 7/1942 Clark et al 1486.24

FOREIGN PATENTS 447,446 5/1936 Great Britain. 787,650 12/ 1957 Great Britain.

ALFRED L. LEAVITT, Primary Examiner. R. S. KENDALL, Assistant Examiner. 

1. A METHOD OF IMPARTING A DECORATIVE STATUARY BRONZE FINISH TO A CUPREOUS SURFACE WHICH COMPRISES TREATING SAID SURFACE WITH POTASSIUM PERMANGANATE, THEREBY FORMING DISCRETE ADHERENT MICROSCOPIC PARTICLES OF PREDOMINATELY CUPROUS OXIDE UNIFORMLY DISTRIBUTED ON SAID SURFACE AND SPACED APART AT A DISTANCE GREATER THAN THE DIMENSION OF SAID PARTICLES, CONTINUING SAID TREATMENT FOR A CONTROLLED PERIOD OF TIME, WHEREBY SAID OXIDE PARTICLES GROW TO COVER A PREDETERMINED FRACTION OF THE TOTAL AREA OF SAID SURFACE, AND THEREAFTER REACTING SAID OXIDE WITH AN AQUEOUS AMMONICAL SULFIDE SOLUTION WHEREBY A DECORATIVE STATUARY BRONZE FINISH IS PROVIDED ON SAID SURFACE WITH A PREDETERMINED DEPTH OF COLOR. 